DESCRIPTION: The applicant and his colleagues showed earlier that a myosin head (S1) can attach either to one or to two actin protomers in an actin filament, depending of whether filament is saturated or unsaturated with S1's. The objective of the project is to test a hypothesis, which was stimulated by this finding, that a major part of a power stroke of skeletal muscle consists of orientational change associated with transition of myosin head from a state in which it binds one actin to a state in which it binds to two actins. In this application they propose to the test this hypothesis by: (i) checking whether the kinetics of binding of S1 to unsaturated filament is consistent with 1 S1 binding to 2 actins; (ii) identifying the amino acid residues involved in making the second contact between S1 and actin; (iii) inducing point mutations in S1 to demonstrate the importance of the second actin binding site; (iv) testing whether cross-bridges in rigor muscle bind to two actins; (v) testing whether the two binding states can be seen during hydrolysis of ATP. The significance of this project is that it presents a critical hypothesis about a fundamental problem in contractility and proposes to test it experimentally. The idea of two binding states and that the transition between them is involved in force generation will be checked in skeletal and smooth muscle, but applies perhaps to the broader category of motor-driven processes. Testing this idea certainly will contribute to the understanding of the molecular changes occurring in acto-S1 during contraction. It is likely that the elucidation of the molecular mechanism of contraction of skeletal and smooth muscle will lead to better modalities in treating diseased muscle.